Electric field is a vector quantity and thus, at a given location in space, a complete determination of the electric field requires specifyinq the direction of the field as well as its magnitude. If the direction of the field is known, a single-axis electric field sensor can be placed with its sensitive direction aligned with the field. However, if the direction of the field is unknown, measurement of the field along all three orthogonal axes (x, y and z) is necessary for a determination of field direction as well as magnitude.
A conventional method for making a complete electric field measurement is to use three single crystal single axis sensors aligned along the three independent spacial directions. Alternatively, Nelson, B., Menzel, C. and DiGuisseppe, T., "Non-Invasive Electro-Magnetic Field Sensor," AFWA-TR-86-3051, Final Report, AF Wright Aeronautical Laboratories, January 1986, describes a single crystal two axis sensor for simultaneously measuring the electric field in two directions. It is suggested that two of these two axis crystals may be aligned in orthogonal directions to enable simultaneous measurement of the electric field in three directions.
The sensor described in Nelson et al. utilizes an electro optic crystal of bismuth germanium oxide Bi.sub.4 (GeO.sub.4).sub.3 (hereinafter BGO). Fiber optic single axis sensors of BGO are described in K. Shibata, "A Fibre Optic Electric Field Sensor Using The ElectroOptic Effect Of Bi.sub.4 Ge.sub.3 O.sub.12," published by IEE, given at the First International Conference On Optical Fiber Sensors, Apr. 26, 1983, and in U.S. Pat. No. 4,631,402 to Nagatsuma et al.
The present invention improves upon the known methods for measuring electric fields by making three axis sensing possible with a single electro-optic crystal. Using the apparatus and method described herein, a measurement of the vector electric field (both direction and magnitude) can be carried out with a single sensing element.